In the making of flexible printing plates, usually a sheet of uncured rubber is placed in overlying relation with a flat rigid mold of thermosetting plastics material and in which is formed a recess of the impression to be printed. The mold and rubber sheet are placed within a press which is heated while the rubber sheet is pressed into the mold to cure the rubber and form the printing plate. A sheet of double face adhesive tape is frequently attached to the back surface of the molded rubber printing plate, and the tape carries a protective covering which is removed when it is desired to attach the plate to a roll or cylinder of a printing press.
While the double face adhesive tape provides for quickly attaching a rubber printing plate to the outer cylindrical surface of a printing cylinder, it is difficult to adjust the position of the plate precisely on the surface of the supporting cylinder. That is, the plate must be pulled or stripped from the cylinder each time it is desired to shift the position of the plate by a fraction of an inch.
Another method of attaching a flexible rubber printing plate to a cylinder is by constructing the cylinder so that it has a magnetized outer surface which attracts a magnetically attractable rubber sheet which is laminated to the back of the rubber printing plate. One such magnetic cylinder is manufactured and sold by the Dayco Corporation, Dayton, Ohio. This company also produces an uncured rubber sheet in which is dispersed ferrous metal particles. This sheet is placed adjacent the sheet of uncured rubber mold material before the latter sheet is cured within a heated press so that the magnetically attractable rubber sheet is laminated to the rubber mold sheet during the molding and curing operation.
It has been found that a number of prolems are encountered in the manufacturing and using of magnetically attractable rubber printing plates. For example, air is frequently entrapped between the uncured rubber mold sheet and the uncured magnetically attractable rubber sheet when the sheets are being laminated together during the molding operation. This entrapped air prevents a continuous bond or lamination of the sheets. There is also a problem of air being entrapped within the recessed impression of the mold. This entrapped air results in pockets being formed within the printing surface of the rubber plate.
It is also difficult to mold the combined rubber sheets to obtain a printing plate of precisely uniform thickness or caliper. When the laminated molded rubber printing plate varies by a few thousandths in thickness, the paper web or other sheet being printed must be pressed against the printing surface of the rubber printing plate with substantial pressure so that the entire printing surface prints effectively. This pressure significantly reduces the useful printing life of the rubber plate.
In some rubber molded plates where the printing surface has fine detail, it has been found necessary to grind the back surface of the plate after it is mounted in an inverted manner on the roll of a grinding machine. This grinding operation must be performed slowly to minimize the springback of the surface as a result of the resiliency of the rubber material. As a result, the grinding operation significantly increases the cost of using a rubber printing plate.
The printing surface of a rubber printing plate is also distorted when the printing plate, which is molded in the flat, is mounted on a printing cylinder. That is, the axially spaced edges of the plate or of the printing surface tend to project above the remaining portion of the surface when the plate is curved to conform to the curvature of the cylinder. The greater the thickness or caliper of the plate and/or the smaller the diameter of the printing cylinder, the more pronounced this deformation becomes. Thus with small diameter cylinders such as those having a three inch or four inch outer diameter, it is frequently desirable to grind the back surface of the plate to achieve a uniform caliper.